The invention relates to an apparatus comprising a holder for a doctor blade and a doctor blade to be fitted in the holder for use in a paper or board machine.
Doctors are used for removing impurities formed on the surface of rolls in paper, board and finishing machines. The doctor is composed of a doctor blade which comes into contact with the roll surface detaching impurities etc, and of a blade holder to which the blade is attached. The holder is in turn attached to a doctor beam arranged on the frame of the machine. In connection with doctoring, air, water and/or chemical jets can be used for enhancing the cleaning process.
The doctor blade is most commonly made of metal. Metal blades suffer from the drawbacks that the roll surface is being coated with doctor metal and the roll surfaces are easily damaged. It is undesirable that the roll surface is being coated with metal because a change in the surface properties of the roll has an unpredictable effect on runnability. For reasons of runnability, even slight damage to the roll surfaces, for example, scratching, is a serious drawback. Metal blades have great problems in doctoring polymeric roll surfaces in particular. In order to overcome these problems, metal blades have been coated with different materials, among other things, with a ceramic. As examples of coated metal blades may be mentioned EP 0 262 137, in which a doctor blade made of carbon steel is coated with molybdenum or a molybdenum alloy by flame spraying, and SE 437 681, in which a doctor blade made of steel is coated with several ceramic coating layers using a plasma or flame spraying method.
In recent years, in addition to metal blades, increasing use has been made of doctor blades manufactured a of fibre-reinforced polymeric material. Such so-called composite blades are disclosed, for example, in U.S. Pat. No. 4,549,933 and in FI patent applications 972776 and 973659. Composite blades have several advantages over metal blades, among other things, light weight, price, freedom of shaping, and gentleness to the roll coating. On the other hand, composite blades wear more quickly than metal blades and leave polymeric material on the roll surface. It has not been possible to coat composite blades with a ceramic or with an equivalent more wear-resistant material because ceramic material does not adhere to a polymeric surface and, on the other hand, polymeric material does not withstand spray coating taking place at high temperature.
Conventionally, the holder of the doctor blade has also been manufactured of steel. Holders have also been made of plastic and of composite materials, but there have been problems in their abrasion resistance. Doctor blades are attached to holders by means of metal rivets or other equivalent members. The blades are replaced frequently, and the metal rivets rub against and wear the holder heavily. In particular, the scratching of the inner surface presents a problem because the scratched surface makes it difficult to fit a new blade in place and, when carbon fibre blades are used, the risk of the blade being damaged because of the scratching of the holder is also obvious.
The object of the invention is to provide a doctor blade and a blade holder as well as an apparatus which are lighter in weight, more resistant to wear and more advantageous.
Characteristic features of the apparatus, doctor blade and blade holder according to the invention are stated in the claims.
It was found that the problems and disadvantages of prior-art arrangements can be avoided by the arrangement according to the invention in which the doctor blade and/or the blade holder is/are made of a composite material on which a ceramic coating has been formed. A composite blade does not scratch the surface of the roll, in contrast to a blade which is coated, for instance, with a ceramic or carbide and whose coating has worn off exposing metal. The composite material may be a fibre-reinforced thermosetting or thermoplastic resin. The ceramic coating can be formed and caused to hold on the composite structure such that the composite structure is provided with an anchor structure of a metal wire, wire mesh, metal foil, a metal powder or equivalent, which anchor structure enables the ceramic to be fixed to the surface of the composite or below the surface if the anchor structure is below the surface. The anchor structure may be on the surface of the composite, partly in the composite structure or totally embedded in the composite structure very close to the surface. The ceramic coating is formed onto the surface of the composite by means of thermal spraying at a temperature that is below 150xc2x0 C., at which the composite does not substantially melt nor is otherwise damaged. There are no limitations on the shaping of the blades or blade holders made of a composite material. The doctor blade and the holder in accordance with the invention can be manufactured of a thermosetting or thermoplastic resin reinforced with fibreglass or carbon fibre, with boron fibre and generally with a fibre composite or their combination. Carbon fibre is particularly preferable since its thermal conductivity is very close to the thermal conductivity of metals. A metal wire, a metal strip, wire mesh, metal foil, a metal powder, combinations of metal and plastic, or a plastic-ceramic powder is/are fixed to the surface of the composite or embedded in the composite. Suitable methods of manufacturing composite blades and holders are casting, compression moulding, pultrusion and extrusion methods and combinations of different methods. The selection of the method is also affected by the orientation of desired fibre directions. The pultrusion method is the most preferable method for its good production rate. On the doctor blade or the blade holder made of a composite material, a ceramic coating is made by means of thermal spraying or by means of electrolytic methods, such as brush electroplating, or by means of autocatalytic methods, such as chemical nickel plating, where ceramic is in dispersion. All conventional ceramics are suitable ceramic materials. However, preferable materials are those which can be readily sprayed, which are low-priced and chemically durable, such as Al2O3 and Cr2O3. The doctor blade may be provided with a ceramic coating either over the entire surface area or only over the wear area depending on where the blade is used. Preferably, one third of the width of the blade is coated. The coating of the doctor blade or the blade holder can be performed with the same ceramic or with a combination of different ceramics, and one or more ceramic layers can be made one above the other. If the composite sections are not completely coated with a ceramic coating, possible wetting of the composite material may be prevented by providing a protective coating on the surface of the composite, for example, electrolytically. The apparatus according to the invention comprising a doctor blade and a holder for the doctor blade can be used in paper and board machine lines, which also include printing machines and coating machines. Optionally, only either the blade or the holder is made of a composite material coated with a ceramic and the other of them is made of metal or another suitable material.
In the following, the invention is illustrated by way of example by means of one advantageous embodiment of the invention, to the details of which the invention is, however, not intended to be confined.